Method and apparatus for cavitational drilling utilizing periodically reduced hydrostatic pressure

ABSTRACT

This specification discloses an improved method and apparatus for drilling a liquid-filled borehole into the earth with cavitational energy. The hydrostatic pressure at the borehole bottom is periodically reduced. Simultaneously, high frequency acoustic energy is imposed in the drilling liquid and cavitation is effected at least during the period of reduced hydrostatic pressure. The shock waves resulting from cavitation in the drilling liquid are utilized for drilling the borehole.

United States Patent 2,893,692 7/1959 Marx 175/56 3,251,424 5/1966Brooks 175/65 3,302,720 2/1967 Brandon 175/56 X 3,315,755 4/1967 Brooks175/56 3,405,770 10/1968 Galle et al.... 175/65 X 3,441,094 4/1969 Galleet al.... 175/56 3,460,637 8/1969 Schulin 175/56 Primary Examiner- DavidH. Brown Attorneys-William J. Scherback, Frederick E. Dumoulin,

William D. Jackson, Andrew L. Gaboriault and Sidney A. Johnson ABSTRACT:This specification discloses an improved method and apparatus fordrilling a liquid-filled borehole into the earth with cavitationalenergy. The hydrostatic pressure at the borehole bottom is periodicallyreduced. Simultaneously, high frequency acoustic energy is imposed inthe drilling liquid and cavitation is effected at least during theperiod of reduced hydrostatic pressure. The shock waves resulting fromcavitation in the drilling liquid are utilized for drilling theborehole.

PATENIEDsEP Hm 3,603'410 sum 1 or 2 PRESSURE u at P'ATENTEDSEP nan SHEET2 [IF 2 FIG. 4

METHOD AND APPARATUS FOR CAVITATIONAL DRILLING UTILIZING PERIODICALLYREDUCED I-IYDROSTATIC PRESSURE BACKGROUND OF THE INVENTION Thisinvention relates to the drilling of boreholes into the earths crust andmore particularly to the drilling of such boreholes by using acousticenergy to effect cavitation and utilizing the resulting shock waves fromthe cavitation induced in a drilling liquid.

Numerous techniques are available for drilling boreholes into the earthscrust. A relatively new drilling technique which is showing increasingpromise utilizes cavitation of a liquid within a borehole. Cavitation isa phenomenon whereby under certain conditions cavities form andviolently collapse within a liquid. A shock wave capable of causingconsiderable mechanical damage to neighboring solid surfaces resultsfrom this cavitation. Cavitation takes place within a liquid when thepressure within the liquid is reduced by a value commonly termedcavitational threshold." The cavitational threshold is a function of thepressure on the liquid and is also influenced by various other factors,such as the presence of gas or solid particles in the liquid which serveas nuclei for cavitation. In general, the value of the cavitationalthreshold of the drilling liquid at a particular location within theborehole is normally somewhat less than the hydrostatic pressure of thedrilling liquid at this location.

Normally in cavitational drilling, a drilling liquid is circulateddownward through a drill string and thence upward to the surface of theearth through the annulus formed between the outer wall of the drillstring and the borehole wall. Cavitation is induced in the drillingliquid by any suitable technique such as by the generation of acousticenergy. The resulting shock waves function to break up the rock surfacesat the bottom of the borehole. The resulting rock fragments areentrained in the circulating drilling liquid and withdrawn from theborehole with this liquid.

Various cavitational drilling techniques are known in the art. Forexample, in U.S. Pat. No. 3,315,755 to W. B. Brooks, there is discloseda technique for carrying out cavitational drilling through the use ofacoustic energy. In this technique a moving piston is vibrated to impartacoustic energy to the drilling liquid. The acoustic energy inducescavitation in the drilling liquid and the resulting shock waves areutilized in drilling the borehole. In another technique disclosed in US.Pat. No. 3,387,672 to E. l... Cook, acoustic energy is utilized toeffect cavitation in the drilling liquid. In this technique, theacoustic pressure required to produce cavitation of the drilling liquidis reduced by introducing into the drilling liquid an amount of gaswhich goes into solution therein.

SUMMARY OF THE INVENTION In accordance with the present invention, thereare provided new and improved methods and systems for practicingcavitational drilling. In practicing the method of the presentinvention, the hydrostatic pressure of liquid in a borehole in thevicinity of the borehole bottom is periodically reduced. Simultaneouslytherewith, an acoustic energy field is produced in the liquid in thevicinity of the bottom of the borehole to effect cavitation in theliquid.

In one embodiment of the invention, this periodic reduction inhydrostatic pressure is achieved by alternately supporting and releasingat least a portion of the drilling liquid above the vicinity of thebottom of the borehole.

In another and preferred embodiment of the invention, the periodicreduction of hydrostatic pressure is effected by producing a secondacoustic energy field within the drilling liquid in the vicinity of theborehole bottom. The frequency of this second field preferably is lowerthan the frequency of the first-mentioned acoustic field.

By another embodiment of the invention, there is provided apparatus fordrilling a borehole by cavitational energy. A first acoustic generatorresponsive to fluid flow is connected to the lower end of a drill stringand a second acoustic generator responsive to fluid flow is connected inthe drill string above the first acoustic generator. The first acousticgenerator is adapted for operating to produce acoustic energy withindrilling liquid in the borehole and the second acoustic generatoradapted for operating to vibrate the first acoustic generator.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration ofa borehole and one system which may be utilized in carrying out thepresent invention;

FIG. 2 is a graph illustrating the pressures imposed in the drillingliquid of a borehole in carrying out this invention;

FIG. 3 is an enlarged cross-sectional view illustrating in detail thedownhole apparatus of FIG. l; and

FIG. 4 is a cross section of apparatus for carrying out a preferredembodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 there is illustrateda system for carrying out cavitational drilling. In utilizing the systemof FIG. ll, drilling liquid is circulated downward through a drillstring and upward through the annulus. The hydrostatic pressure at thebottom of the borehole is periodically reduced by alternately supportingand releasing at least a portion of the drilling liquid above thevicinity of the bottom of the borehole. Simultaneously, an acousticgenerator is actuated to produce an acoustic field in the drillingliquid at a downhole location to effect cavitation in the drillingliquid.

More particularly, and with reference to FIG. 1, borehole l is shownextending into the earths crust 2. A conduit, or drill string, 3 extendsfrom the surface of the earth into borehole 1. Drill string 3 isconnected at the surface to a source of drilling liquid by any suitablemeans such as a stuffing box (not shown). The drilling liquid II iscirculated downward through drill string 3 to the vicinity of the bottomof borehole I and upward through annulus 13 formed between the outerwall of drill string 3 and the wall of borehole l. Drilling liquid 11 isconducted from annulus 13 of borehole l by conventional means (notshown) to a mud pit. A 1ift should 5 which functions as an externalsupport means is connected to drill string 3 at a downhole location.Examples of external support means which may be used are annular ringsor packers adapted for supporting at least a portion of the drillingliquid in annulus 113. An acoustic generator 9 is supported by drillstring 3 in the lower end thereof. A lost motion connection means 7 isoperatively interposed drill string 3 intermediate lift shoulder 5 andacoustic generator 9. A surface lifting means, pump jack I5, isconnected by cable 17 to drill string 3.

In operation, pump jack 15 periodically raises and lowers drill string 3to alternately support and release at least a portion of drilling liquid11 by lift shoulder 5, thereby periodically reducing the hydrostaticpressure of drilling liquid II at a downhole location in the vicinity ofthe bottom of borehole ll. Simultaneously therewith, acoustic generator9 is actuated by flow of drilling liquid ll therethrough to generate anacoustic energy field within drilling liquid 11 in the vicinity of thebottom of borehole 1. Lost motion connection means 7 allows liftshoulder 5 to be raised and lowered without moving acoustic generator 9from the vicinity of the borehole bottom. Cavitation is induced in thedrilling liquid by the combined effects of the acoustic energy field andthe reduced hydrostatic pressure at least within the time periods ofreduced hydrostatic pressure.

Referring to FIG. 2, there is seen a typical pressure versus time plotillustrating the pressure variations induced in a drilling liquid by themethods of this invention. The static hydrostatic pressure isrepresented by horizontal line P The pressure at which cavitation takesplace in the drilling liquid at a particular location in the boreholesis represented by horizontal line P The fluctuating hydrostatic pressureproduced by embodiments of this invention is illustrated by curve P Thefluctuating pressure produced by the relatively high frequency acousticenergy field is illustrated by curve P,. Curve P, is shown forillustrative purposes as being superimposed upon curve P and thusrepresents the composite effect of the low and high frequency acousticfields in the drilling liquid. It is seen that the frequency of theacoustic energy field illustrated by curve P, is high as compared to thefrequency of the periodic reduction of hydrostatic pressure shown bycurve P Hereafter, P, may be referred to as a high frequency field and Pmay be referred to as a low frequency field. It is to be understood thatthe terms high frequency and low frequency are used in a comparativesense only as relative to one another.

Preferably, the ratio of the frequency of P, to the frequency of Pshould be greater than 5. By such ratios, problems of phaserelationships between P, and P are avoided and a reduction ofhydrostatic pressure by the composite of the acoustic fields representedby P, and P is insured.

It will be recalled that, in the case of drilling liquids in a borehole,cavitation is effected when the pressure in the drilling liquid isreduced by a value somewhat less than the value of the hydrostaticpressure at a particular location in a borehole. The intensity of theshock waves resulting from cavitation increases with increasing pressureon the liquid. Thus, in cavitation drilling, a relatively highhydrostatic pressure is desirable from the standpoint of the amplitudeof the produced energy. However, as the pressure increases, so does thecavitational threshold and, thus, and increasing amount of energy inneeded in order to induce cavitation. Accordingly, when utilizing agiven acoustic generator to induce cavitation, there is a limit to thedepth at which cavitation can be induced. The present invention isdirected to extending the depth at which cavitation can be produced by agiven acoustic generator. The depth is extended by periodically reducingthe hydrostatic pressure in the drilling liquid in the vicinity of theborehole bottom while simultaneously producing a high frequency acousticfield to effect cavitation in the drilling liquid.

With further reference to FIG. 2, it is seen that under staticconditions the value of the amplitude of acoustic energy required toproduce cavitation in a liquid-filled borehole is P,,-P:, =P which isthe cavitational threshold under these conditions. At the time of themaximum negative fluctuation of P the hydrostatic pressure is seen to bereduced to a value of P To produce cavitation at this time it is onlynecessary to further reduce the pressure in the drilling liquid by avalue of P P P This is contrasted to the larger value P regiired toproduce cavitation under static hydrostatic pressure. Thus, underconditions illustrated by FIG. 2, cavitation takes place during the timeperiod of t,t whereas without periodically reducing the hydrostaticpressure cavitation would not be induced.

Referring now to FIG. 3, there are seen further details of the downholeportion of FIG. 1. Acoustic generator 9, illustrated as an acousticsiren, it attached by threaded sleeve 19 to lost motion connector 7which serves to connect drill string 3 to acoustic generator 9 by meanswhich allow lift shoulder to be reciprocated relative to acousticgenerator 9. Lost motion connector 7 is comprised of a cylindricalhousing 14 which is closed at its upper end about drill string 3 in amanner which allows drill string 3 to slide therethrough. An annularshoulder 10 is attached to the lower outer portion of drill string 3.Annular shoulder 10 strikes the upper shoulder 12 of cylindrical housing14 and prevents drill string 3 from being pulled from the housing. Inthis manner, longitudinal motion is allowed by lost motion connector 7between lift shoulder 5 and acoustic generator 9, thus allowing liftshoulder 5 to be longitudinally oscillated without moving acousticgenerator 9 from its position with respect to the borehole bottom.

The acoustic generator 9 is comprised of an outer case 33, a mud turbine23, a stator 27, and a rotor 29 connected operatively one with theother. Mud turbine 23 is attached to outer case 33 by a spider 24.Stator 27 is secured within the lower portion of outer case 33 and rotor29 is held by shaft 31 in operable relationship with stator 27. Inoperation, drilling liquid 11 is pumped through drill string 3, hollowshaft 21, mud turbine 23, ports 25, passages 26 of stator 27, andpassages 28 of rotor 29, and back up through annulus 13 to the surface.The acoustic siren is thus actuated by the fiow of drilling liquid 11 toproduce a high frequency acoustic field. Outer case 33 terminates in aborehole standoff 34 which extends below rotor 29, thus protecting rotor29 from the boreholes bottom.

In FIG. 4 there is illustrated a system for carrying out cavitationaldrilling utilizing two acoustic generators positioned in the drillstring. Drilling liquid is circulated downward through the drill stringand upward through the annulus between the drill string and the boreholewall. The uppermost acoustic generator is actuated by the circulatingdrilling liquid to generate low frequency acoustic energy which istransmitted down the drill string and vibrates the lowermost acousticgenerator, thereby vibrating the radiating face of the piston of thelowermost generator and producing a low-frequency acoustic field in thedrilling liquid. Simultaneously, the lowermost acoustic generator isactuated by the circulating drilling liquid to vibrate at a highfrequency the piston of the lowermost generator and produce a highfrequency acoustic field in the drilling liquid. Cavitation is thuseffected in the drilling liquid by the combined effects of the low andhigh frequency acoustic fields in the drilling liquid.

More particularly with reference to FIG. 4, a portion of borehole l isseen extending into the earths crust 2. The lower portion of drillstring carrying vibration generator 35 and mud turbine 37 which drivespiston 52 is seen in borehole 1'.

The vibration generator 35 is actuated by flowing drilling liquid 11 togenerate low frequency acoustic energy in the drill string. The lowfrequency acoustic energy is transmitted downward through the drillstring and adapter 36 and vibrates mud turbine 37 the drill to the lowerend thereof. Simultaneously, the mud turbine is actuated by flowingdrilling liquid 1 1 to vibrate piston 52 which is operatively connectedtherewith to produce a high-frequency frequency acoustic field in thedrilling liquid in the vicinity of the borehole bottom. Since mudturbine 37 itself is also being vibrated by the low-frequency acousticenergy produced by vibration generator 35, piston 52 is vibrated by acomposite of the high-frequency acoustic energy produced by mud turbine37 and the low-frequency acoustic energy produced by vibration generator35 to produce the composite highand low-frequency acoustic fields in thedrilling liquid illustrated by FIG. 2. The face 53 of piston 52 thusserves as a radiating face for both the mud turbine 37 and vibrationgenerator 35.

Vibration generator 35 is suspended by its upper end to the lower end ofdrill string 80 and is connected through adapter 36 to mud turbine 37and is lowered to the vicinity of the bottom of liquid-filledborehole 1. Vibration generator 35 is vibrated at a low frequency,thereby vibrating mud turbine 37 and piston 52 at said low frequency. Inthis manner, radiating face 53 of piston 52 imposes a low frequencyacoustic field in drilling liquid 11 at the borehole bottom.Simultaneously therewith, mud turbine 37 is actuated to vibrate piston52 at a high frequency. Thus, radiating face 53 of piston 52 serves asthe radiating face or transducer for both the vibration generator 35 andmud turbine 37. The low frequency acoustic field produced by vibrationgenerator 35 serves to periodically lower the hydrostatic pressure inthe drilling liquid in the vicinity of the borehole bottom. The highfrequency acoustic field produced by mud turbine 37 is superimposed onthe low frequency acoustic field and effects cavitation in the drillingliquid at least during a portion of the negative cycle of the lowfrequency acoustic field.

A suitable vibration generator 35 for use in this combination isdescribed in U.S. Pat. No. 3,554,005, to A. G. Bodine, Jr. Such avibration generator is adapted to be driven by circulating drillingliquid and is comprised of a plurality of oppositely rotatingeccentrically weighted rotors mounted for rotation on axes transverse ofthe drill string to generate alternating forces along a directionlongitudinally of said drill string. As seen in FIG. 4, vibrationgenerator 35 is comprised of a cylindrical case 55 having a fluidpassageway 57 passing therethrough connected in fluid communicationwithin drill string 80. Two heavy eccentrically weighted rotors 59, 61are mounted within fluid cavities 63, 65 in cylindrical case 55 in fluidcommunication with fluid passageway 57. Rotors 59, 61 are mounted onseparate axes passing transversely through cylindrical case 55 andoffset equal distances in opposite directions from the longitudinal axisof fluid passageway 57. Flexible blades 67 are connected with rotors 59,61 so that when drilling liquid 11 passes through vibration generator 35rotors 59, 61 are rotated in opposite directions. The flow of drillingliquid is adjusted so that the two rotors fall into a mode or rotationsuch that the centers of gravity of the two rotors move up and down instep with one another but oscillate laterally in opposition to oneanother. Thus, lateral vibrations are canceled while vertical vibrationsare added to produce longitudinal vibrations within the drill string.

A mud turbine suitable for use in this combination is described in U.S.Pat. NO. 3,315,755, to W. B. Brooks. Such a mud turbine is adapted to beresponsive to fluid flow through a drill string and comprises a pistonand a fluid turbine motor operatively connected to the piston toreciprocate the piston at a frequency and magnitude such that when thepiston is spaced from the bottom of the borehole an acoustic energyfield is generated in the drilling liquid to effect drilling of theborehole. As seen in FIG. 4, mud turbine 37 is connected by threadedconnector 36 to vibration generator 35. Mud turbine 37 is comprised ofouter case 70 which houses a multielement fluid turbine rotor 72. Thefluid turbine rotor 72 is connected by a series of shafts and gears topiston 52 having radiating face 53. Fluid communication is providedthrough mud turbine 37 and piston 52 whereby liquid may be circulatedtherethrough to longitudinally vibrated piston 52 and produce a highfrequency acoustic field in the drilling liquid in the borehole.

Iclaim:

l. A method of cavitationally drilling a borehole having a liquidtherein into the earths crust comprising:

periodically reducing the hydrostatic pressure of said liquid in thevicinity of the borehole bottom; and

simultaneously introducing an acoustic field in said liquid in thevicinity of said borehole bottom with an acoustic generator positionedin the vicinity of said borehole to effect cavitation in said liquid.

2. The method of claim 1 wherein the frequency of said periodicreduction of hydrostatic pressure is less than the frequency of saidacoustic field.

3. A method of cavitationally drilling a borehole into the earth's crustcomprising:

positioning an acoustic generator in the vicinity of the boreholebottom; circulating a drilling liquid downward through a drill string insaid borehole and upward through the annulus defined by said drillstring and the wall ofsaid borehole;

periodically reducing the hydrostatic pressure of said drilling liquidin the vicinity of said borehole bottom; and

simultaneously operating said acoustic generator to produce an acousticfield in said drilling liquid in the vicinity of said borehole bottom toeffect cavitation in said drilling liquid.

4. The method of claim 3 wherein said periodic reduction of hydrostaticpressure is effected by producing a second acoustic field within saiddrilling liquid in the vicinity of the borehole bottom.

5. The method of claim 4 wherein the frequency of said first-namedacoustic field is greater than the frequency of said second acousticfield.

6. The method of claim 5 wherein the ratio of the frequency of saidfirst-named acoustic field to the frequency of said second acousticfield is greater 5. 5.

7. A method of cavitationally drilling a borehole into the earths crustcomprising:

circulating a drilling liquid downward through a drill string in saidborehole and upward through the annulus defined by said drill string andthe wall of said borehole;

alternately supporting and releasing at least a portion of drillingliquid above the vicinity of the bottom of said borehole; and

simultaneously producing an acoustic field in said drilling liquid inthe vicinity of said borehole bottom of effect cavitation in saiddrilling liquid. 8. A method of cavitationally drilling a borehole intothe earths crust comprising:

circulating a drilling liquid downward through a drill string in saidborehole and upward through the annulus defined by said drill string andthe wall of said borehole;

periodically reducing the hydrostatic pressure of said drilling liquidin the vicinity of the borehole bottom by generating acoustic energy insaid drill string at a point above the vicinity of said borehole bottomand transmitting said energy down said drill string to the vicinity ofsaid borehole bottom thereby producing a first acoustic field withinsaid drilling liquid in the vicinity of said borehole bottom; and

simultaneously producing a second acoustic field in said drilling liquidin the vicinity of said borehole bottom to effect cavitation in saiddrilling liquid.

9. The method of claim 8 wherein said second acoustic field is generatedby a radiating face and said acoustic energy generated in said drillstring is transmitted to the radiating face.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3609 410 Dated September 7. 1971 Inventor(s) Frank A Angona It iscertified that error appears in the aboveidentified patent and that saidLetters Patent are hereby corrected as shown below:

' Column 2, line 44, change "should" to --shoulderline 49, after "drillstring 3", "in" should be --at--; line 51, after "interposed" insert--in--; line 75, "boreholes" should be --borehole--.

Column 3, line 31, "and increasing" should read --an increasing-- line32, "in needed" should read -is needed--; line 45, "P -P3 =P should be-P -P =P line 59, "it attached"should be -is attached+ Column 4, line12, "boreholes" should be -borehole-;

line 37, "The low frequency" should be -This low frequency--; line 39,after "turbine 37" delete "the drill" and insert --connected--; line 42,cancel "frequency" (second occurrence).

Column 5, line 49, after "borehole" insert --bottom--.

Column 6, line 22, "greater 5.5." should be --greater than 5.

line 28, after "portion of" insert -said-; line 32, "of effect" shouldbe --to effect--.

Signed and sealed this 7th day of March 1972.

(SEAL) Abtess:

EDWARD M.FLETCHIR,JR.. ROBERT GOT'I'SCHALK Attosting Oificer'Commissioner oi Patents

2. The method of claim 1 wherein the frequency of said periodicreduction of hydrostatic pressure is less than the frequency of saidacoustic field.
 3. A method of cavitationally drilling a borehole intothe earth''s crust comprising: positioning an acoustic generator in thevicinity of the borehole bottom; circulating a drilling liquid downwardthrough a drill string in said borehole and upward through the annulusdefined by said drill string and the wall of said borehole; periodicallyreducing the hydrostatic pressure of said drilling liquid in thevicinity of said borehole bottom; and simultaneously operating saidacoustic generator to produce an acoustic field in said drilling liquidin the vicinity of said borehole bottom to effect cavitation in saiddrilling liquid.
 4. The method of claim 3 wherein said periodicreduction of hydrostatic pressure is effected by producing a secondacoustic field within said drilling liquid in the vicinity of theborehole bottom.
 5. The method of claim 4 wherein the frequency of saidfirst-named acoustic field is greater than the frequency of said secondacoustic field.
 6. The method of claim 5 wherein the ratio of thefrequency of said first-named acoustic field to the frequency of saidsecond acoustic field is greater
 5. 5.
 7. A method of cavitationallydrilling a borehole into the earth''s crust comprising: circulating adrilling liquid downward through a drill string in said borehole andupward through the annulus defined by said drill string and the wall ofsaid borehole; alternately supporting and releasing at least a portionof drilling liquid above the vicinity of the bottom of said borehole;and simultaneously producing an acoustic field in said drilling liquidin the vicinity of said borehole bottom of effect cavitation in saiddrilling liquid.
 8. A method of cavitationally drilling a borehole intothe earth''s crust comprising: circulating a drilling liquid downwardthrough a drill string in said borehole and upward through the annulusdefined by said drill string and the wall of said borehole; periodicallyreducing the hydrostatic pressure of said drilling liquid in thevicinity of the borehole bottom by generating acoustic energy in saiddrill string at a point above the vicinity of said borehole bottom andtransmitting said energy down said drill string to the vicinity of saidborehole bottom thereby producing a first acoustic field within saiddrilling liquid in the vicinity of said borehole bottom; andsimultaneously producing a second acoustic field in said drilling liquidin the vicinity of said borehole bottom to effect cavitation in saiddrilling liquid.
 9. The method of claim 8 wherein said second acousticfield is generated by a radiating face and said acoustic energygenerated in said drilL string is transmitted to the radiating face.